Fibrodysplasia ossificans progressiva (FOP) is a genetic disease which causes cartilage tissues or bone tissues to be ectopically formed in soft tissues, such as skeletal muscle, tendon, and ligament, where bone tissues are not normally formed (Non Patent Literatures 1 to 3). In this disease, ectopic ossification occurs throughout the entire body including the face so that ectopic bones and existing normal bones are fused to remarkably reduce the range of joint motion or to deform the body (Non Patent Literatures 1 to 3).
The ectopic ossification in FOP proceeds chronically with growth, whereas acute ectopic ossification is also known which proceeds while manifesting a symptom called flare-up caused by muscle injury, viral infection, or the like (Non Patent Literature 1). The flare-up is swelling with inflammatory response or sustained pain as principal symptoms. The flare-up is induced by bruise, falling, intramuscular injection, or the like, which causes muscle injury. In addition, sudden flare-ups with no clear cause are also known. For FOP, invasive medical acts such as biopsy and operation are contraindicated because ectopic bones are formed after flare-up. Thus, the ectopic bone tissues cannot be surgically removed. The ectopic bone tissues in FOP are consisted of normal cartilage cells or osteoblasts and bone metabolismee of heterotopic bones is the same as normal bone tissues. As such, only the ectopic bone tissues cannot be medically removed using drugs or the like.
Any fundamental therapy for suppressing the ectopic ossification in FOP has not yet been established, and only symptomatic treatment for pain or the like has been made. Thus, the ectopic bone tissues formed in FOP are very difficult to remove, and the development of a promising drug that can exert prophylactic effects before the onset of ectopic ossification has been expected.
Activin like kinase 2 (ALK2) gene encoding a receptor of bone morphogenetic proteins (BMPs) that induces ectopic bone formation in soft tissues including skeletal muscle tissues has been identified as a responsible gene for FOP(Non Patent Literature 4). The ALK2 gene is identical to activin A type I receptor 1 (ACVR1) gene. ALK2 having an amino acid substitution has been found from familial and sporadic FOP cases (Non Patent Literature 4).
Human or mouse ALK2 is a single-pass transmembrane protein consisting of 509 amino acids and having a signal peptide and functions as a transmembrane type of serine-threonine kinase receptor binding to BMPs (Non Patent Literatures 1 to 3). ALK2 binds to BMPs at its N-terminal extracellular region and activates the downstream intracellular signal transduction system through an intracellular serine-threonine kinase. BMP receptors are classified based on their structures and functions into 2 types: type I receptors including ALK2; and type II receptors (Non Patent Literatures 1 to 3). The type II receptors are constitutively active enzymes that exhibit kinase activity even if not bound with BMP. On the other hand, the type I receptors including ALK2 are inactive enzymes in a state unbound with BMP and exhibit kinase activity in a manner dependent on binding to BMP. This is probably because upon binding to BMP, type II receptor kinase phosphorylates type I receptor intracellular domain as the substrate, which can change its conformation, and activates the type I receptor (Non Patent Literatures 1 to 3).
Type I receptors are known to beconstitutive active independent of a type II receptor by substitution of a particular amino acid in the intracellular region (Non Patent Literatures 1 to 3). Overexpression of this constitutively active mutants of the type I receptors activate the intracellular signal transduction system without BMP stimulation. Thus, the type I receptors are considered as responsible molecules that transduce BMP signals from the outside to the inside of cells.
A mutation in ALK2 identified from familial and typical sporadic FOP patients was the R206H mutation in which Arg206 is substituted by His (Non Patent Literature 4). All of gene mutations previously identified from FOP cases have been reported to cause amino acid substitutions in the intracellular region of ALK2. Most of these mutations from FOP cases reside mainly in or around the ATP-binding region in the intracellular domain of ALK2 (Non Patent Literature 5).
Overexpression of the ALK2 mutants identified in FOP in cultured cells activates the intracellular signal transduction system of BMP without BMP stimulation (Non Patent Literature 6). Accordingly, for example, small molecular inhibitors for ALK2 kinase, RNAi or exon skipping methods which specifically suppress the expression of genetically mutated ALK2, downstream transcriptional factor inhibitors of ALK2 receptor, and inhibitors of BMP signal-mediated osteoblast differentiation are being developed as therapeutics for FOP (Patent Literature 1 and Non Patent Literatures 1 to 3).
All of small molecular compounds or nucleic acids currently under development as therapeutics for FOP are expected to inhibit intracellular ALK2 signal after they penetrate cell membranes. However, any effective drug delivery method has not yet been established for nucleic acid drugs. In addition, there still remain problems such as the low specificity of kinase inhibitors for other ALK receptor families with high similarity to ALK2. As such, it has been desired to develop novel therapeutics for FOP having high specificity for ALK2. Antibody drugs that act on the extracellular region of ALK2 and inhibit its signal are highly safe therapeutic approach that utilize the physiological immune system. The antibody drugs facilitate stable drug delivery via blood flow and are capable of specifically inhibiting ALK2 without acting on other ALK receptor families with high identity thereto. Furthermore, the antibody drugs can be expected to have inhibitory effect on wild-type ALK2 and various intracellular mutant ALK2s including novel unidentified mutants. The antibody drugs are expected not to inhibit ALK3 expressed in cells throughout the body. Therefore, unlike general osteoblast differentiation inhibitors, antibodies specifically inhibiting ALK2 can be expected to serve as drugs less likely to influence the growth, maintenance, and regeneration of normal skeletal tissues. However, any antibody that specifically binds to ALK2 and exhibits therapeutic effects on FOP has not been known so far.